1. Field of the Invention
The invention relates to an antenna for a radar unit for level measurement and more particularly to an antenna for a radar unit for level measurement with a hollow guide system and dielectric rod radiator.
2. Description of Related Art
Antennas for level measurement devices that operate using microwaves have been known for some time and are used for continuous level measurement of both liquids and poured goods, for example. The measurement principle is based on transmitting short microwave pulses from a high frequency radiator (HF radiator). In a combined transmission and reception system, the pulses reflected from the contents are detected and the travel time of these pulses is measured to determine the distance to the contents.
The coupling of microwaves inside the container is accomplished by means of a suitable HF radiator. It is desirable to make sure that no temperature or corrosion sensitive components of the antenna are located in the container.
When using such level-measuring devices that operate with microwaves, it is necessary to guarantee a good seal between the interior of the container and the electronic part of the antenna. This is especially important when the level in containers such as industrial tanks is to be determined, in which overpressures or vacuums prevail at high or low operating temperatures and the container holds explosive and/or highly corrosive and/or toxic media. Consequently, the antenna should be designed to be as resistant as possible to temperature, pressure, and corrosion. Moreover, the antenna should conform to the regulations governing approval for use in facilities where there is a risk of explosion. It is useful if the antenna conforms to the harmonized European standards "Electrical Operating Media for Explosion-Risk Areas" and ElexV Ex-Zone 0 as well as Ex-Zone 1. Ex-Zone 0 covers areas in which dangerous explosive vapors are present constantly or for a long time, while Zone 1 covers areas in which dangerous explosive vapors are to be expected to occur occasionally.
High-gain antennas are used to direct microwaves, bundled as tightly as possible, against the surface of the contents. In this case, the divergence, i.e. the aperture angle, of an antenna beam is inversely proportional to its size/wavelength ratio. As a result, at a given wavelength and with various design boundary conditions, aperture angles from several degrees to approximately 40.degree. constitute dimensions that are in conventional use.
A dielectric rod radiator is a type of antenna that is frequently in use in level measurement using microwaves because of its high resistance to chemicals. Dielectric rod radiators and their function are described, for example, in the publication "IEEE Transactions on Antennas and Propagation," Volume AP-30, No. 1, January 1982, pp. 4-58. Dielectric rod antennas consist essentially of a hollow guide into which HF energy is fed by a suitable HF-coupling device and at whose end a preferably tapering rod made of dielectric material is located.
A dielectric rod antenna of this kind is shown in German patent application 94 12 243.1 of the Applicant, and is intended especially for use in a radar device for level measurement. High resistance to chemicals, resistance to pressure, and resistance to corrosion are achieved by virtue of the fact that the dielectric rod antenna is coated on its side facing the container with a corrosion-resistant protective layer that is permeable to electromagnetic waves, preferably a layer of enamel, plastic, or ceramic. This known design permits mounting the dielectric rod antenna in relatively thin tubular stubs while maintaining the required small aperture angle.
It has been found that the previous design and assembly expenses for dielectric rod antennas of this type used for level measurement, especially as far as mounting on a container is concerned, are relatively high and therefore cost-intensive. Specifically, a conventional rod antenna uses a flange of its own with bolt openings provided therein for bolting it to a matching flange on the container lid that is especially provided for that purpose. When the rod antenna is installed, it is attached in a fixed position on the container and therefore can not be rotated relative to the container. This is particularly problematic if it is determined after installation that the rod antenna should be rotated around its own lengthwise axis to filter out disturbing echoes.
It is desirable to improve on the known antenna for a radar unit used for level measurement and equipped with a dielectric rod radiator such that the antenna can be mounted more simply on a container and hence an antenna that costs less to mount can be used.